1. Field of the Invention
The present invention relates to gas diffusion electrodes, catalysts, and electrolytic cells suitable for chlor-alkali electrolysis, metal-air batteries, fuel cells and the devices concerned with electrochemical oxygen reduction, such as oxygen sensors.
2. Background Art
Gas diffusion electrodes are used as oxygen cathodes suitable for chlor-alkali electrolysis, metal-air batteries, and fuel cells.
A gas diffusion electrode has a multilayer structure composed of a gas diffusion layer, a reaction layer, and a current collector for electrical connection. Gas phase oxygen is exposed to the gas diffusion layer. The reaction layer resides between the gas diffusion layer and an electrolyte. After passing through the gas diffusion layer, oxygen is consumed through a reduction reaction on an oxygen reduction catalyst in the reaction layer. The reaction proceeds according to the following equation:O2+2H2O+4e−=4OH−
The gas diffusion layer is required to allow the oxygen to pass therethrough rapidly and to diffuse uniformly into the entire reaction layer. The gas diffusion layer is also required to prevent the electrolyte from permeating to the gas phase. The gas diffusion layer is comprised of a material formed of carbon particles bonded to each other with a material, such as polytetrafluoroethylene, having high water repellent properties. The gas diffusion layer must also conduct electrons from the current collector to the reaction layer.
The reaction layer contains uniformly dispersed oxygen reduction catalyst particles in electronic continuity with the gas diffusion layer and current collector. In the reaction layer, a large interface area is formed among the oxygen, electrolyte, electrons, and the oxygen reduction catalyst.
The particles for forming the reaction layer can be prepared, for example, by mixing carbon particles, intermixed with an electrode catalyst with a dispersion of a fluorinated resin such as polytetrafluoroethylene, dispersing the mixture thus formed using a dispersing agent such as an alcohol, filtrating and drying the dispersion thus formed, and pulverizing the dried material into fine particles. Methods for processing the gas diffusion electrode layers are disclosed by Furuya in U.S. Pat. Nos. 6,630,081 and 6,428,722.
The current collector may be, for example, a wire mesh or a foam material, which is composed of nickel, silver, or the like.
Mainly noble metals such as platinum and silver, either dispersed in or supported on carbon black in the reaction layer, have been used and investigated as oxygen reduction catalysts for concentrated alkaline solution.
In the chlor-alkali application, utilization of an oxygen consuming cathode instead of a hydrogen evolving cathode provides the opportunity of reducing the specific power requirement since the theoretical operating voltage is lower by approximately 1.23V.
However, the over-voltage of electrochemical oxygen reduction gas diffusion cathodes using previous catalysts is high or increases over time. In the case of industrial chlor-alkali electrolysis, if the over voltage is too high, economical advantages of electrolyzers using gas diffusion cathodes are not realized compared to conventional electrolyzers using hydrogen cathodes. Accordingly, a stable catalyst with high catalytic activity for electrochemical oxygen reduction is needed to lower the over-voltage. Such a catalyst may also provide benefits for oxygen cathodes used in non chlor-alkali applications.
T. Hyodo et al. has shown Pr0.6Ca0.4CoO3 perovskites to outperform Pt as an oxygen reduction catalyst in alkaline electrolyte, at least for 200 hours where air is the oxidant. Similarly, M. Hayashi et al. have shown La0.8Rb0.2MnO3 to outperform Pt for 100 hours. Kudo et al. reported that a Nd0.8Sr0.2Co1−yNiyO3 catalyst electrode was stable over 300 hours with a potential >−50 mV relative to a Hg/HgO reference electrode at a presumably low current density and for Nd0.8Sr0.2CoO3 at 100 mA/cm2 the potential was −150 mV, and −250 mV for Nd0.8Ca0.4CoO3. G. Karlsson demonstrated −200 mV at 150 mA/cm2 with Nd0.2Ca0.8MnO3. Yuasa et al. reported approximately −60 and −65 mV relative to a Hg/HgO reference electrode at 400 mA/cm2 current density for a very short test of 3 hours in duration for La0.8Sr0.2MnO3 and La0.8Sr0.2Mn0.8Fe0.2O3. T. Hyodo et al. also claimed Fe in the perovskite composition aided stability when running La0.5Sr0.5FeO3 approximately −225 mV relative to Hg/HgO reference electrode at 325 mA/cm2 current density for 90 hours.
Thus many perovskite-type oxides have been shown to have high catalytic activity, where the over-voltage of gas diffusion electrodes using these catalysts was lower than that of gas diffusion electrodes using platinum or silver. However, neither the stability of perovskite-type oxides in a concentrated alkaline solution nor the long-range durability of gas diffusion electrodes using perovskite-type oxides have previously been confirmed, especially at high current density.